The present invention relates to a pressurized fluid brake system for vehicles, particularly road vehicles, of the type wherein the brake pedal pressure is assisted by an auxiliary force and transmitted to the wheel brake cylinders via pressure fluid lines in which braking pressure modulators are inserted to regulate the braking pressure. Such brake systems are generally equipped with transducers for the direct and/or indirect measurement of each wheel's rotational behavior and the vehicle's velocity. These systems also employ electronic circuits which process and logically combine the measured values in order to generate control signals for the braking modulators.
Numerous brake systems of this type are already known for preventing locking of all or particular wheels. Typically, they have control electronics which monitor constantly measured wheel rotational behavior and vehicle velocity, or their timevariations. Upon detecting that locking is imminent, the control electronics reduce the braking pressure at the wheel concerned by means of the modulators, maintaining this reduced pressure constant and then increasing it again at an appropriate time. This adjusts the wheel brake slip to a value favorable for slowing down the vehicle while maintaining driving stability and steerability. As for adherence of the wheels to the road, the obtainable coefficient of friction as well as the forces occurring at the wheel during braking depend on a great many parameters. Because the most important of these parameters vary widely, such antilocking or brake slip control systems are relatively complicated in design. This is a disadvantage because when a vehicle is driven cautiously its brake slip control apparatus seldom takes up work, except in emergency or panic stops. Because of this, it is rarely apparent whether the antilocking protection is working. Therefore, the antilocking apparatus has to be checked by additional devices at specific intervals, e.g. at each start-up of the vehicle's motor, or at certain preset times, so possible errors will be recognized in time.
When engineering the performance specifications of such brake systems, another problem is the adaptation of the brake force distribution to the static and dynamic loads on the vehicle axles. Conventional brake force distributors are limited to an invariably adjusted, pressure-responsive control. Load-responsive or deceleration-responsive brake force regulators are likewise known in several variants. But all of them permit only a relatively rough approximation to the axle load distribution, and at most in one of the two limit conditions, "unloaded" or "loaded".
Likewise, a computerized brake force distributor system is known in which with the vehicle at a standstill the static axle load distribution is measured by sensors and input to an onboard microcomputer. The microcomputer uses these measured static axle load values and the value of the braking pressure to control the brake force distribution in the front and rear axle circuits pursuant to a formula stored in a memory device (European patent application EP-A1 062246). Such a brake force distributor has the disadvantage that the microcomputer uses only a calculated value of the braking friction between the wheels and the road at the front and rear axles, not the actual friction values prevailing during braking. Yet such actual friction values depend on a great number of parameters and can affect the brake force distribution. Therefore, to reliably preclude dangerous overbraking of the rear axle, the designer generally cautiously engineers the brake system so that in the majority of cases the contribution of the rear axle wheels to the vehicle braking is small. Furthermore, the actual wheel-to-road friction values occurring in practical operations have been found to differ widely from the nominal values arrived at by preset calculations.
The brake characteristic values assumed as constant when engineering and dimensioning the brake force distribution and the mathematical formula stored in the computer's memory are in practice subject to considerable variation. For example, variation arises from manufacturing tolerances, aging of parts, contamination, changes of the spring constants, adjustment defects, and changes in temperature, etc. For this reason, it has been proposed that the actual brake slip at the front and rear wheels be determined with the aid of wheel and vehicle sensors together with logic circuits. Then the brake slip at the rear axle can be made dependent upon the brake slip at the front axle so that during braking the coefficient of friction occurring at the rear wheels will be about, or somewhat lower than, that of the front wheels (German Pat. No. 3301948).
Conventional brake force regulators are typically designed so that in the majority of cases first the front wheels, and only then the rear wheels, will lock when overbraked. This is because the locking of the rear wheels brings about a high risk of skidding while locking of the front wheels reduces steerability. The loss in steerability due to front wheel locking is considered the lesser of two evils in comparison to an imminent danger of skidding. It is true that usually the major brake load occurs on the front axle on account of the dynamic axle load distribution. But if the braking forces at the rear axle are too low or the potentially available coefficient of friction at the rear wheels is underutilized, the stopping distance will be correspondingly longer under certain conditions, e.g. on slippery roads. Moreover, when the brake pedal pressure is inceased too heavily beyond a minor pedal pressure on a slippery road, none of the brake force distributors described is able to prevent both front and the rear wheels from locking, causing the vehicle to lose both its steerability and driving stability.
Therefore, an object of the present invention is to overcome these disadvantages by providing a brake system which, with comparatively little effort, optimally distributes the brake force between the front and rear wheels under all conditions to be met in practical operations. Such a system acheives a uniform exploitation of available friction at both the front and rear axles and prevents locking of the wheels. It also reliably precludes the imminent danger of skidding and loss of steering ability.